1. Field of the Invention
The present invention relates to an image forming apparatus of electrophotographic method in which a latent image is visualized employing colored particles such as a toner in a printer, a facsimile or a copying machine, and more particularly to a color image forming apparatus having a plurality of image forming means.
2. Description of the Related Art
One type of the color image forming apparatuses is a tandem system in which image forming means including of a photoconductor, a charger, an exposure unit, a developing unit, and a cleaner is provided for each color of black (K), yellow (Y), magenta (M) and cyan (C), and a toner image formed by each image forming means is transferred onto the paper or an intermediate transfer body to form a color image.
In the case where a color image is produced by mixing a plurality of color toners subtractively, the hue of the final color image may be changed if a gradation density curve of each color toner image is varied. Therefore, it is required to keep the gradation density curve stable for each image forming means in the color image forming apparatus of tandem system. The variable factors of the gradation density curve include a variation in the developing gap, a change in the exposure sensitivity of photoconductor, a change in the film thickness of photoconductor, the charge amount of developer, and a change in the resistance of developer.
A conventional method has been well known in which prior to the image formation, a test patch latent image is formed on the photoconductor by the exposure unit, and developed by the developing unit to form a test patch image, a reflection density (image density) of the test patch image is measured by an optical image density sensor, and if the reflection density (image density) is deviated from a specified value, a developing bias, a grid voltage of the charger, and the supply of toner are controlled (refer to JP-A-63-240569).
A method for correcting for a variation in the developing electric field is well known, including detecting a potential on the surface of photoconductor by a potential sensor, detecting a film thickness of photoconductor in some way, changing a light amount of laser to keep the developing electric field constant, and controlling the potential on the surface of photoconductor, for example, as described in JP-A-11-15214.
However, with the conventional technique as above described, the image density of a solid area (solid image portion) that is mainly developed due to a parallel electric field may be corrected, but a variation in the gradation density curve caused by a variation in the peripheral electric field can not be corrected, resulting in a problem that the hue of color image may be varied for each apparatus or with the passage of time.
Conventionally, a method has been offered in which when the hue of color image is changed, the hue of an output test pattern is read into the control portion of the image forming apparatus by a scanner, and the raster expansion of input image data is modified to correct for a variation in the hue of the color image (refer to JP-A-2001-358955).